Research on innovative design of thermostat products based on MCU

With the global economic integration, product competition among countries is becoming increasingly fierce. Whoever holds the initiative in the market will be able to remain invincible. This requires relevant enterprises to quickly respond to market demand and innovate products.
　　Product innovation is based on potential market demand, which is reflected in the innovation of the functions, principles, overall layout, specific structure and other aspects of existing products in the design stage. Only a few designs are completely new. Therefore, it can be said that the core of product design is innovation, and the design process is an innovation process. The purpose of product design is to use. Only products that are more convenient for people to use can generate profits and benefit mankind in certain areas.
　　Based on the increasingly widespread demand for small household appliances and the fierce market competition at home and abroad [1-5], this paper has made an innovative design for the thermostat, an important component in small household appliances, and successfully used it in electric kettles, meeting the market's diversified demand for electric kettles.
1 Current status of the electric kettle and thermostat market
　　In the global electric kettle market, one of the most commonly used important components is the mechanical thermostat. The principle is as follows:
　　when water is heated to boiling, a large amount of water vapor is generated, which is transferred to the metal sheet of the mechanical thermostat through the steam pipe. The metal sheet expands and contracts with heat and cold, and the deformed metal sheet pushes the mechanical structure of the thermostat to do work, thus disconnecting the circuit switch, thus completing the water boiling process. After a period of time, the metal sheet returns to its original position as the temperature drops, so that the water can be heated again.
　　Most of the thermostats in the global electric kettle market come from the United Kingdom, which has layers of patent barriers and makes most of the profits [6-7]. Among these brands, STRIX, OTTER, etc. are the main market leaders. Generally, electric kettles of well-known domestic and foreign brands mostly use STRIX and OTTER thermostats. This type of thermostat has the following disadvantages:
　　(1) The kettle lid must be kept closed when boiling water. Only when a large amount of water vapor is kept from dispersing into the air during the boiling process can the fluidity of water vapor be used to transfer a large amount of heat to the metal sheet of the thermostat through the steam pipe. Otherwise, the water will remain in a boiling state and the power will not be automatically cut off. This not only wastes electricity, but also affects the quality of water, shortens the life of the kettle, and has a bad impact on consumers.
　　(2) When a kettle of water is boiled and needs to be continued, it takes a while for the thermostat to reset the switch, which will give consumers the illusion that the kettle switch has lost its function and does not conform to the fast characteristics of the kettle.
　　(3) There are certain requirements for the water level during operation, and the general minimum water level is 0.5L.
　　(4) It is difficult to have the opportunity to expand functions, such as temperature display, temperature control, automatic water cooling, etc.
2 Product Innovation Points
　　In order to get rid of the dependence on mechanical thermostats, facing foreign technology monopoly and opportunities in domestic and foreign markets, and targeting the shortcomings of mechanical thermostats, the innovation of this design lies in: using MCU control technology and using thermistors as temperature sensors to achieve safe, practical and convenient temperature control. Its functions are as follows:
　　(1) Real-time water temperature display, display range: 0℃~110℃.
　　(2) Users can set the temperature by pressing buttons.
　　(3) Temperature control accuracy: The temperature is maintained at ±3℃ of the set temperature.
　　(4) Music prompts when the water temperature reaches the set temperature.
　　(5) Anti-dry boiling function.
　　The principle block diagram of the innovatively designed thermostat is shown in Figure 1. The control circuit is based on the Samsung S3F9454 MCU. This chip is a low-power, high-performance single-chip 8-bit CMOS MCU with a built-in 10-bit precision A/D module, 4KB of on-chip Flash memory and 208B of RAM. The Flash memory can be repeatedly programmed, which is convenient for software modification and upgrading.

Thermostat principle: Place the thermistor at a certain position of the electric kettle, sample the temperature through the thermistor, and transmit the temperature-related information here to the MCU for processing; according to the regular relationship between the temperature here and the temperature of the water in the kettle, the MCU compares the calculated water temperature with the temperature set by the user. If the water temperature is greater than or equal to the set temperature, the MCU controls the disconnection of the heating circuit to stop the circuit from heating the heating plate. Similarly, if the temperature of the heating plate read by the thermistor is greater than 110°C (that is, when dry burning occurs), the heating circuit of the heating plate will be disconnected immediately.
3 Overall layout scheme design
3.1 Scheme 1: Measure steam temperature to achieve temperature control
　　The temperature sensor transmits the measured steam temperature to the S3F9454MCU through the steam port. The MCU analyzes the signal from the temperature sensor to decide whether to supply power to the heating load and output the working status. When the temperature signal measured by the sensor is greater than or equal to the set temperature, the heating stops and the circuit is reset. This scheme realizes water temperature control by sensing the steam temperature. Since the measurement of steam temperature is relatively easy, it takes less time. This scheme is suitable for users who only need to boil water. The schematic diagram of the mainboard and control panel for realizing temperature control is shown in FIG2 .

3.2 Solution 2: Measuring the temperature of the kettle body to achieve temperature control
　　Place the thermistor temperature sensor on the side of the kettle body (in order to avoid modifying the structure of the kettle body, it is best to place it at the junction of the handle and the kettle body). The MCU is programmed according to the regular relationship between the kettle body temperature and the water temperature to achieve automatic temperature control. However, there are certain requirements for the placement structure of the thermistor. The place on the kettle body that contacts the thermistor needs to be thinner than the wall thickness of other places, and this place needs to be sealed to prevent it from being affected by the air temperature; there needs to be a medium between the thermistor and the water on the kettle body to prevent the kettle water from being contaminated. This solution can control the water temperature arbitrarily, so it is suitable for users with multiple water temperature requirements.
3.3 Solution 3: Measuring the temperature at the bottom of the heating plate to achieve temperature control
　　A heating wire is used on the heating plate at the bottom of the kettle body. The place where the heating wire is connected to the power cord is called the cold end because it generates less heat, and the other parts are called the hot end because they generate more heat. The thermistor is placed at the cold end of the heating plate at the bottom of the kettle body. According to the experimental data, the relationship between the water temperature and the temperature of the heating plate is obtained. The MCU is programmed according to this relationship, so that the temperature of the heating plate measured by the thermistor can be converted into the temperature of the water and displayed on the display screen. At the same time, the temperature of the heating plate can be detected to prevent dry burning. This solution has low requirements for the overall structural design and is more convenient to realize the anti-dry burning function.
　　The above three solutions can meet the personalized requirements of different users to meet the needs of the serialized design of electric kettles. The following takes the third solution as an example to optimize the system structure design.
4 Structural optimization design
4.1 Determination of the placement of the thermistor
　　The key step to ensure the accuracy of the measurement is to find an optimal position to place the thermistor. The actual test was carried out at different points for 1000W, 800W power, 1.7L, and 1.0L water level. The test content and results are shown in Table 1 and Figure 3.

As can be seen from Table 1 and Figure 3, placing the thermistor near the cold end of the heating plate is closest to the actual water temperature.
　　The thermistor placement structure design is shown in Figure 4. [page]
4.2 Relationship between thermistor and water temperature and heating plate temperature
　　When the thermostat measures the temperature through the thermistor, the MCU is programmed according to the corresponding relationship between the water temperature, the thermistor measured temperature, and the temperature of the thermistor installation on the heating plate, and performs data processing to control the heating circuit. Actual
　　measurement method: Under the voltage of 220V±10%, different water volumes (1.0L, 1.2L, 1.5L, 1.7L) are used, the thermistor is placed at the bottom of the heating plate, and the thermocouple is placed at the thermistor installation location, the thermistor installation location in the plastic tube, and the kettle water. The timing starts from the water temperature of 40℃, and the temperature patrol meter is used to print data every 10 seconds. The heating ends when the water temperature reaches 90℃, and then continues to print until the water temperature begins to drop.
　　Through actual measurement, the relationship between the thermistor placed on the heating plate at the bottom of the kettle and the water temperature and the temperature of the heating plate was obtained: when 242V voltage and 1.0L water volume were selected, the actual measured results are shown in Figure 5; when 242V voltage, 1.2L, 220V voltage, 1.5L, 198V voltage, 1.2L, and 1.7L were selected, the actual measured results are shown in Figure 6.

From the comparison of the above figures, it can be seen that the water temperature, the actual temperature measured by the thermistor, and the temperature at the location where the thermistor is installed on the heating plate show regular changes: whether it is cold water or hot water, whether the heating plate is heated or not, the water temperature > the actual temperature measured by the thermistor > the temperature of the heating plate, and the temperature of the water in the kettle is 7±3℃ higher than the temperature of the heating plate at that location. The MCU will be programmed according to these rules.
5 Software Design
　　The main program flow chart is shown in Figure 7. After the system is powered on, the MCU circuit is powered on, the program starts running, and the system is initialized: first, it detects whether a key is pressed. If a key is pressed, the user-set temperature is read in and displayed; if no key is detected, the set temperature uses the default value of 100℃, and the real-time temperature is displayed. Start heating, and when the temperature reaches the set temperature, stop heating and a music prompt is played. The anti-dry-burning function is completed by the interrupt program. When the temperature of the heating plate reaches 110℃, an interrupt is generated, and the MCU immediately disconnects the heating circuit. The
　　innovatively designed thermostat has the following advantages over the mechanical thermostat:
　　(1) The water temperature can be set arbitrarily, and accurate control of the water temperature can be achieved.
　　(2) There is no minimum water level limit when boiling water.
　　(3) The system functions can be expanded, such as adding a timing function.
　　(4) The size and shape of the circuit board can be designed according to the structural requirements to meet the requirements of electric kettles of various shapes.
　　(5) It can meet the personalized needs of different users.
　　The thermostat design based on Samsung MCU has gotten rid of the dependence on the metal sheet of the mechanical thermostat. After actual testing, the performance indicators of the prototype have met the design requirements, and the thermostat can also be used in other small household appliances, with broad market prospects.
References
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